In 1993, Hamers-Casterman and his colleagues discovered a specific type of antibody in camelids, i.e. heavy chain antibodies (HCAbs) that naturally lack light chains. A single domain antibody (sdAb) consisting of only one heavy chain variable region is obtained by cloning its variable region. Its crystal structure is elliptical, 2.5 nm in diameter and 4 nm in height. So it is also called Nanobody (Nb; 15 kDa) which is the smallest functional antigen-binding fragment at this stage. Nanobodies have many unique properties compared with conventional antibodies: 1) Nanobodies are highly homologous to the human VH family 3 and 4 and therefore have little immunogenicity; 2) Nanobodies have a small molecular weight of only about 15 kDa and a simple structure which facilitate them to be expressed in microorganisms in a large scale and easy to be purified; 3) Nanobodies can recognize a large number of epitopes, including epitopes hidden in the molecular cleft; 4) Nanobodies can easily penetrate the tissue due to the small molecular weight to reach the site which is difficult to be reached by conventional antibodies; 5) Nanobodies are highly soluble and stable at denaturing or high temperature environments.
Human CD7 molecule is a cell surface glycoprotein having a molecular weight of about 40 kDa and belongs to the immunoglobulin superfamily. CD7 molecules are mainly expressed on the surfaces of most of the thymocytes, 85% of the peripheral blood T lymphocytes and natural killer cells. Although current studies have shown that the specific function of CD7 molecules is not yet clear, but experiments show that the T lymphocytes of CD7-deficient mice react normally to stimulation and the cell growth and proliferation are not effected when the antibody binding CD7 molecules on human T lymphocytes. At the same time, an important property of the CD7 molecule is that when it binds to its antibody, it starts endocytosis immediately. On the basis of this important property, several studies have been made by conjugating immunotoxins to CD7 molecules for target delivery to human leukemia and lymphoma cells thereby achieving the purpose of treating diseases and coupling immunotoxins to treat acute graft-versus-host diseases. These experiments have been carried out in clinical trials. At the same time, it has been studied to treat HIV infection by conjugating protein to CD7 molecules for targeted delivery siRNAs to T lymphocytes. However, these experiments were performed on single-chain antibodies (scFv; 30 kDa) isolated from conventional antibodies. Single-chain antibodies are difficult to invade into tissues and cells due to their relatively large molecular weight while nanobodies have small molecular weight. Meanwhile, single-chain antibodies are difficult to be expressed in prokaryotic expression system in a soluble form. Nanobodies are easy to be expressed in prokaryotic expression system in a soluble form and easy to be renatured. Therefore, preparing human CD7 nanobodies for disease treatment may be one of the alternatives with more effective and lower cost.
At present, there is no research report on the specific nanobodies directed against human CD7 epitopes.